Web-based XML Editing with W3C XML Schema and XSLT, Part 2
Pages: 1, 2, 3
MetaXSL+
What we need is an XSLT stylesheet which takes our XSD and transforms it into the corresponding XSL+. The concept applied is the same as the concept used for the generation of XSLGUI. The stylesheet which generates the XSL+ is called MetaXSL+ here.
Figure 3. XML Schema to XSL+ processor
The basic task of the MetaXSL+ is as follows:
- For every element make an
xsl:templatewith thematchattribute equal to the name of the element. - If
the element is a datatype element, add it and the functions needed to
produce the value of the element to this
xsl:template - If the element is complexType, add the
xsl:apply-templates select="*"function - For every element make an
xsl:templatewith thematchattribute equal toinsert-elementNamewhere elementName is the name of the element. - If the element is complexType, add each child element (recursively) N times where N is the minOccurs of the child element
Creating new instance documents
We saw that the XSL+ is able to transform an
insert-elementName tag into the corresponding element with
the correct body. Now making an instance of Person.xsd is nothing else but
a insert-person tag in a document (bootstrap). All we need is
an XSLT which finds the first element of our XSD (person) and creates an
XML+ document which consists of only one line namely:
<insert-person>The following XSLT does the trick:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsl:output method="xml" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:template match="xsd:element">
<xsl:variable name="inserttag"
select="concat('insert-', @name)"/>
<xsl:element name="{$inserttag}"/>
</xsl:template>
</xsl:stylesheet>Using the same cycle as described above this new XML+ document will be transformed into an instance document using the XSL+. The result will be
<xsl:element name="person">
<xsl:element name="name"/>
<xsl:element name="date_of_birth"/>
<xsl:element name="phone"/>
</xsl:element>which is an initial instance of Person.xsd.
Inserting elements which do not appear in the instance document
The question now is how can we add an element which does not appear in
the new instance document (an element with "minOccurs=0") such as the
course in a new person instance or the
course_code in a new course element.
There are different approaches possible:
- When making a new instance document, add all the elements to the document, including elements with a minOccurs equal to zero. This approach is an easy way out, and even though the created instance document is a valid document, it is not what we expect: an empty element is not equal to not having that element in the instance document.
- Adjust the XSLGUI so that it generates plus signs after
each element. In our person example we would have a plus sign for
courseaftername, one afterdate_of_birthand one afterphone. This way if the user clicks on either of the first two plus signs the course element will be inserted at a place which is not allowed according to our XSD. Aninsert-courseafter the phone element will be valid. For an instance document with a huge number of elements this approach can be frustrating for the user as there will probably be lots of attempts before the user can find a valid location. - Find a way through trial and error (validation) to know exactly where plus signs are meaningful and give that knowledge to the XSLGUI stylesheet. Conceptually it is an elegant solution but technically it is quite an effort to implement.
- Another solution and the one chosen in this article is to produce plus signs for the children of a complexType element at that level as follows. For all the children, if the child has a minOccurs equal to 0, and if the number of element occurrences in the document is equal to 0, produce a plus sign at the level of the complexType element containing the children.
In our example person is a complexType and its child
course has a minOccurs equal to 0. The XSLGUI will
be generated as follows:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:strip-space elements="*"/>
<xsl:template match="person">
<xsl:param name="root">
/person
</xsl:param>
<xsl:variable name="index">
1
</xsl:variable>
<html>
<head>
<title>person</title>
</head>
<body>
<form name="xmsForm" action="xms.InputPage"
method="post">
<!-- The output of the following will look
like:
<a href="javascript:submitForm(
'/person[1]/course',
'insertfirstposition');"
onClick="submitForm(
'/person[1]/course',
'insertfirstposition');
return false;">
Add course +
</a>
-->
<xsl:variable name="courseCount"
select="count(course)"/>
<xsl:if test="$courseCount='0'">
<a>
<xsl:attribute name="href">
<xsl:value-of
select="concat('javascript:submitForm(',
"'",'/person[',
$index,']','/course',
"'",',',
"'",
'insertfirstposition',
"'",');')"/>
</xsl:attribute>
<xsl:attribute name="onClick">
<xsl:value-of
select="concat('submitForm(',
"'",'/person[',
$index,']','/course',
"'",',',
"'",
'insertfirstposition',
"'",');
return false;')"/>
</xsl:attribute>Add course +
</a>
</xsl:if>
<xsl:apply-templates>
<xsl:with-param name="path"
select="$root"/>
</xsl:apply-templates>
<input type="submit" name="action"
value="save"/>
</form>
</body>
</html>
</xsl:template>
...
</xsl:stylesheet>Now at the person level (top level), if the user clicks on the plus
sign to insert a course, a request will be sent to the server with the
action insertfirstposition and the XPath parameter will be
/person/course. We know that the new instance document does
not contain any course elements (otherwise we would not get a plus sign
for course), so there is no way we could know where to insert
a course element. All we know is that it has to be inserted at
/person. Then, beginning at the first position in
/person, insert the course and validate the
document. If valid, we will use the XSLGUI to produce the GUI;
otherwise, we will insert the element at the next position till we find a
valid position for the course. This approach inserts the
element at the first allowed location in the original instance
document. Note that it might be desirable to insert the element not at the
first possible location but somewhere else. This could become an issue in
very complex combinations of elements.
The XUpdate document used to insert the new element at different positions is as follows:
<?xml version="1.0" encoding="ISO-8859-1"?>
<xu:modifications
xmlns:xu="http://www.xmldb.org/xupdate">
<xu:append select="/person[1]"
child=index>
<xu:element name="insert-course"/>
</xu:append>
</xu:modifications>Where index starts at 0 and increases by one till there is a valid position found.
Conclusion
This article described a concept in which elements can be inserted into an XML instance document through an auto-generated, form-based GUI, based on the XML Schema of the instance document and XSLT. The capability of editing and inserting or removing elements using the corresponding XML Schema makes it a complete and functional approach for the implementation of an XML web-based editor.
Resources
- Support for choose option
2006-04-03 14:12:02 Ram_Peri - MetaXSLgui doesn't work?
2003-08-19 16:35:46 Anthony Hope - MetaXSLgui doesn't work?
2003-08-20 01:10:09 Anthony Hope - a generic solution
2003-08-11 05:18:37 Ali Mesbah - a generic solution
2003-07-29 08:49:39 Erik Ostermueller - a generic solution
2003-08-11 05:20:47 Ali Mesbah - very nice
2003-06-27 16:07:13 Eric Hanson - very nice
2009-01-15 12:00:57 nico_deb
